This invention relates to colloid stabilized latices which are useful as pressure sensitive adhesives or as laminating adhesives.
The replacement of solvent borne adhesives with aqueous latices is a continuing trend in industry for reasons of air quality, safety, and economics. However, the transition has not been simple and is by no means complete. Aqueous adhesives have found limited acceptance, being regarded as generally inferior to organic solution adhesives either in performance or coating rheology or both. The poor coating rheology typical of conventional latex adhesives is manifested in several forms: first, there is a tendency in roll application to form ridges parallel to the direction of coating. Ridges adversely effect adhesive performance by reducing the effective area of contact with the secondary substrate during bonding, but more importantly they substantially detract from the appearance of a lamination particularly when a clear film is used. Even if the primary substrate is opaque, ridges in the underlying adhesive may prevent subsequent satisfactory printing on a label or decal for example. A second rheological deficiency of conventional latices is their tendency to exhibit a decrease in viscosity at the shear rates encountered in high speed coating operations, leading to poor substrate wetting, reduction in coating weight, and requiring a reduced line speed with attendant loss of productivity.
An additional deficiency of latex adhesives is the tendency for coagulum to build up on the metering roll when conventional emulsions are applied by a reverse roll coater. Such coagulum may arise either from poor mechanical stability under shear or from imperfect doctoring leading to a thin coating on the roll which dries and cannot be redispersed so that these particles eventually transfer to the applicator roll and mar the appearance of the coating.
The rheological defects of latex pressure sensitive adhesives prepared with conventional surfactants arise at least in part from the need to incorporate thickeners such as hydroxyalkyl cellulose or polyacrylic acid. Without thickening, the low viscosity conventional latex will not completely wet a silicone coated release liner. This is a prerequisite for coating employing the transfer method which is generally preferred particularly for heat sensitive face stocks. Even for direct coated self-wound tapes, thickening may be required depending on the nature of the coater and the porosity of the substrate.
In the particular case where the adhesive is to be used to form laminates wherein at least one of the surfaces is a printed surface, the presence of any residual surfactant can lead to discoloration or bleeding of the ink. This is recognizably a problem in applications such as overlaminating of books or printed labels where the purpose of the outer surfacing film is to preserve the integrity of the printed surface. It is also known that the presence of surfactant reduces the wet bond strength and water resistance of the laminate.